1. Field of the Invention
The present invention relates to a picture signal encoding and decoding apparatus which encodes a digital picture signal for transmission and decodes a received encoded signal.
2. Description of the Prior Art
FIG. 1 is a simplified block diagram of a configuration of a picture signal encoding and decoding apparatus of a conventional type, for example, as shown in "Interframe Encoding apparatus for video conferencing", a technical report IE84-4, 1984 of The Institute of Electronics and Communication Engineers of Japan. In the figure, 1 is an encoding section which encodes a digitized picture signal series; 2 is a variable-length-encoding section which variable-length-encodes code words encoded in the encoding section 1; 3 is a transmitting buffer which smooths out speed; 6 is a framing section; 7 is a deframing section; 8 is a receiving buffer; 9 is a variable-length-decoding section; 10 is a decoding section. 101 is a digitized picture signal series; 102 is a code-word series encoded in the encoding section 1; 103 is a code-word series variable-length-encoded in the variable-length encoding section; 108 is a feedback control signal; 109 is a code-word series whose speed is smoothed out by the transmitting buffer 3; 110 is a received code-word series; 111 is an output read from the receiving buffer 8; 112 is a code-word series converted to a fixed-length code-word series in the variable-length decoding section; 113 is a digital picture signal series decoded in the decoding section 10. FIG. 2 is an illustrative drawing showing the operation of the transmitting buffer 3. FIG. 3 is an illustrative drawing showing the operation of the receiving buffer 8.
The operation is explained in the following. The digitized picture signal series 101 is converted to a code-word series 102 in the encoding section 1. In this step, each code word has a fixed length. Next, the variable-length encoding is performed by utilizing the deviation of occurrence probability of each code word in the variable-length encoding section 2; thus the code-word series 102 is converted to a variable-length code-word series 103. In the result, the sum of the length of the variable-length code-word series 103 becomes smaller than a code quantity, the sum of the length of the code-word series 102, so that transmission efficiency can be improved. As the code quantity becomes variable, to send out these data to a channel at a constant transmission rate smoothing out of the speed is performed in the transmitting buffer. The output 109 of the transmitting buffer 3 has a constant rate (a code quantity in a unit time) corresponding to the transmission rate. The operation of the transmission buffer 3 is explained referring to FIG. 2. In FIG. 2(a), the axis of abscissa represents time and the axis of ordinate represents a buffer accumulation quantity. The series to be input to the transmitting buffer 3 are variable-length and the output has a constant rate; therefore the accumulation quantity varies as shown in FIG. 2(a). FIG. 2(b) is a buffer output corresponding to FIG. 2(a). The decrease in the buffer accumulation quantity means a state where data are read from the buffer more than the data are written to the buffer, and when the accumulation quantity becomes zero nothing is left to be read in the buffer so that a dummy signal is output. In contrast with this, when the writing to the buffer is more than the reading from the buffer accumulation quantity increases. As the buffer capacity is limited, if the buffer accumulation quantity is kept increasing, the buffer may overflow in time. To prevent the overflow, when the buffer accumulation quantity becomes large, the creation of code words is suppressed by controlling the operation of the encoding section 2 by using a feedback control signal 108. The output 109 read from the buffer has a constant rate. In a framing section 6 framing is performed at fixed intervals for the transmitting buffer output 109 and the data are output to a transmission line. On the receiving side deframing is performed for an input signal series in a deframing section 7, and the data are once stored in a receiving buffer 8. The output of the buffer 8 is a variable-length code series 111, and the series 111 is converted to the fixed-length code series 112 in the variable-length decoding section 9. The fixed-length code series 112 is decoded in the decoding section 10 and the digital picture signal series 113 is obtained. In the above-mentioned process of receiving and decoding of a signal, the decoding section 10 can only decode a certain section of a picture signal series (for example, a sequential picture signal series such as a picture signal frame or a line) at a fixed speed. Therefore, the speed of the fixed-length code series 112 cannot exceed the speed in which the decoding section 10 is capable of decoding. In the similar way, there is a speed limit in the operation of the variable-length decoding section 9. On the transmitting side, the transmitting buffer 3 located near the output port smooths out the speed of a signal; in correspondence to this, the receiving buffer 8 on the receiving side adjusts the speed. The operation of the receiving buffer 8 is explained in the following referring to FIG. 3. It is assumed that the input signal series 110 for the receiving buffer 8 is given as shown in FIG. 3(a). As the signal series is of a variable-length code-word series, when the series is separated, for example, into picture frames, the intervals are not uniform. T in FIG. 3(b) is a time interval in which the variable-length decoding section 9 and the decoding section 10 can process code words corresponding to the frame. When the quantity of the code words corresponding to a frame in FIG. 3(a) is smaller than the time interval in FIG. 3(b), the code words corresponding to the next frame sent in a time difference mentioned in the above are accumulated in the receiving buffer 8. If a condition under which data accumulates continues long, there is a risk that data may overflow the buffer memory, and the capacity of the receiving buffer is therefore designed to have some redundancy.
A picture signal encoding and decoding apparatus of a conventional type is constituted as described above; it is therefore necessary to control the operating speed of a variable-length decoding section and a decoding section as fast as possible. Because of this, there has been a problem that the size of the apparatus and the capacity of the receiving buffer have to be made large, which makes the delay time large.
The encoding of a signal in an encoding section of a picture signal encoding and decoding apparatus is, to be precise, performed by a transform coding method.
Transform coding is a process wherein a digital picture signal is transformed to a sequence corresponding to a spatial frequency by orthogonal transformation such as by a method in which a discrete cosine transform is used as shown, for example, in the following reference. W. H. Chen. "Scence Adaptive Coder". (IEEE. Transactions on Communications, Vol. COM 32, No. 3, March, 1984.) FIG. 4 is a block diagram showing the configuration of a transform coding apparatus as shown in Kato, et al., "A proposal for an encoding control method in MC - DCT encoding system" (No. 203 all-Japan meeting of information and system branch of The Institute of Electronics, Information and Communication Engineers of Japan, 1987.) In the figure, 31 is a subtracter which performs subtraction of an interframe predictive signal 202 for movement compensation from an input signal 201; 21 is an orthogonal transformation section which performs an orthogonal transformation for an interframe differential signal 203; 22b is a quantizing section which threshold-processes and quantizes a transformed coefficient 204 obtained by orthogonal transformation according to a buffer accumulation quantity 211; 23 is an inverse orthogonal transformation section which creates a decoded interframe differential signal 207 by performing inverse orthogonal transformation for a quantized output signal 206; 32 is an adder; 24 is a frame memory which creates the interframe predictive signal for movement compensation 202; 25 is a movement compensation section; 26 is a variable-length encoding section; 27 is a transmitting buffer.
The operation is explained in the following. A differential signal 203 from which a redundant component is removed is created by getting the difference between the digitized input signal 201 and the interframe predictive signal for movement compensation. In the orthogonal transformation section 21, the transformed coefficient 204 is created by transforming the interframe differential signal to a spatial frequency domain through orthogonal transformation. In the quantizing section 22b, the transformed coefficient Ci 204 undergoes threshold processes as mentioned below based on a buffer accumulation quantity to be described later.
Buffer accumulation quantity: large.fwdarw.threshold value Th: large,
buffer accumulation quantity: small.fwdarw.threshold value Th: small,
Th&lt;Ci.fwdarw.Ci: a significant coefficient,
Th.gtoreq.Ci.fwdarw.Ci: an insignificant coefficient.
In the case of a threshold process, a transformed coefficient 204 which is classified to be a significant coefficient is quantized in the quantizing section and is output as a quantized output signal 206. On the other hand, the transformed coefficient 204 which is classified to be an insignificant coefficient is output as a zero quantized output signal 206. The quantized output signal 206 is encoded into a variable-length signal together with a moving vector 205, which is explained later, in the variable-length encoding section 26 and is output as an encoded data 210; on the other hand the signal 206 is converted to a decoded interframe differential signal 207 in the inverse orthogonal transformation section 23. In the adder 32, the decoded interframe differential signal 207 is added to the interframe predictive signal for movement compensation 202 to create a decoded signal 208. The decoded signal 208 is stored temporarily in the frame memory 24, and in the case of movement compensation the interframe predictive signal for movement compensation 202 is output. In the movement compensation section 25, the movement quantity of the input signal 201 is detected by using a foreframe decoded signal 209 from the frame memory 24 to output a movement vector 205. The transmitting buffer 27 stores the encoded data 210 temporarily and outputs them as transmitting data 212 at a constant bit rate, and at the same time it outputs a buffer accumulation quantity 211 as a feedback signal to prevent buffer overflow.
The encoding section of a transform coding portion of a conventional picture signal encoding and decoding apparatus is constituted as described above, so that the adaptive quantization of a input signal according to its statistical characteristic is difficult, and it has thus been a problem to compress the signal effectively.